Strong at heart, can help in laying a foundation (4,4)
I believe the answer is:
hard core
'help in laying a foundation' is the definition.
I can't judge whether this defines the answer.
'strong at heart' is the wordplay.
'strong' becomes 'hard' (similar in meaning).
'at' means one lot of letters go next to another.
'heart' becomes 'core' (synonyms).
'hard'+'core'='HARD-CORE'
'can' acts as a link.
Can you help me to learn more?
(Other definitions for hard core that I've seen before include "filthy" , "Pornography" , "Rubble used in road foundations" , "Why some apple is uneaten" , "Filling rubble" .)